1. Technical Field
This disclosure relates to a sheet processing device and an image forming apparatus, and more specifically to a sheet processing device including a sheet path switching mechanism and an image forming apparatus including the sheet processing device.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, an inkjet recording apparatus is known that uses a recording head (liquid-droplet ejection head) for ejecting droplets of ink.
Such inkjet recording forming apparatuses fall into two main types: serial-type image forming apparatuses that form images by ejecting droplets from the recording head while moving a carriage mounting the recording head in a main scanning direction, and line-head-type image forming apparatuses that form images by ejecting droplets from a linear-shaped recording head held stationary in the image forming apparatus.
For example, in a case where the image forming apparatus employing the liquid-ejection recording method is an equipment-type inkjet printer used for bookbinding or small-volume production of printed matters, the equipment-type inkjet printer may have multiple inkjet heads of thermal-type or piezoelectric-type for high-speed driving. Without moving the heads by, e.g., a carriage in a direction perpendicular to a sheet conveyance direction, the equipment-type inkjet printer can form images while the sheet is conveyed.
Inkjet recording apparatuses are widely used because of advantages of, e.g., high speed, less noise, less constraint in the available type of recording media, and ease of colorization.
As one image formation mode, image forming apparatuses may have a duplex printing mode to perform image formation on both front and back faces of a sheet of recording media. In the duplex printing mode, after a recording unit finishes image formation on a first face, i.e., a front face of the sheet, the sheet is reversed and conveyed to the recording unit again. Then, the recording unit forms an image on a second face, i.e., a back face of the sheet.
Conventionally, to switch the sheet conveyance direction between a route to output the sheet after image formation on the first face and a route to reverse the sheet, for example, JP-2005-148365-A and JP-2002-316758-A propose to provide a pivotable switching plate at a position at which the two routes converge.
JP-2005-148365-A proposes to provide a switching plate that can select an orientation to set the conveyance direction of the sheet fed from a sheet feed device to a direction toward an attachment belt opposing the recording unit or an orientation to set the conveyance direction of the sheet separated from the attachment belt after image recording to a direction toward a reversal conveyance unit. In JP-2005-148365-A, each orientation of the switching plate is set by a driving unit, such as a solenoid.
Recently, there has been an increased demand for reducing the processing time for image formation. To reduce the processing time, for example, enhancement of the sheet conveyance speed or reduction of the conveyance intervals between sheets have been employed. However, the above-described configuration in which the switching plate is driven by a solenoid does not meet the demand for reducing the processing time because a time lag occurs at the switching operation.
Hence, as a mechanism not requiring a driving source to switch the sheet conveyance direction, for example, JP-2002-316758-A proposes to provide elastic sheet members, such as mylar sheets, at the position of the switching plate to prevent the sheet having passed a first conveyance path from returning to the first conveyance path and guide the sheet to a second conveyance path. In the configuration described in JP-2002-316758-A, the elastic sheet members are arranged in the width direction of the sheet and have a single-support beam shape in which ends of the elastic sheet members are urged to overlap, i.e., contact ribs of a conveyance guide disposed at a side opposite a side at which base ends of the elastic sheet members are mounted.
Using the elastic sheet members of JP-2002-316758-A can cope with a delay of operation caused in the case in which the solenoid of JP-2005-148365-A is used. However, in the case where the elastic sheet members of JP-2002-316758-A are used instead of the switching plate of JP-2005-148365-A, the following problem may occur.
For example, in the case where the elastic sheet members of JP-2002-316758-A are used instead of the switching plate of JP-2005-148365-A, the front ends of the elastic sheet members contact the attachment belt.
In such a case, as described in JP-2005-148365-A, when the sheet is conveyed to the attachment belt, the front ends of the elastic sheet members are pushed by the front end of the sheet and separated from the attachment belt. As a result, the sheet is guided toward the attachment belt. When the sheet attached on the attachment belt moves to the reversal conveyance path, the sheet is separated from the attachment belt and guided to the reversal conveyance path by utilizing a contact of the front ends of the elastic sheet members with a surface of the attachment belt.
As described above, to guide the sheet to the reversal conveyance path, the front ends of the elastic sheet members need to contact the surface of the attachment belt. Consequently, the surface of the attachment belt is rubbed by front edge portions of the elastic sheet members, thus damaging and deteriorating the belt surface.
For the elastic sheet members of JP-2002-316758-A, both faces of each elastic sheet member are used as guide faces to guide the sheet. For example, a first face of each elastic sheet member is used to guide the sheet to the reversal conveyance path, and a second face of each elastic sheet member is used to feed the sheet from the reversal conveyance path. When the first face is used as the guide face, the front ends of the elastic sheet members contacting a guide portion are pushed and spread so as to separate from the guide portion. By contrast, when the second face is used as the guide face, the contact state of the front ends of the elastic sheet members with the guide portion is maintained so that the sheet can pass over the second faces of the elastic sheet members opposite the first faces contacting the guide portion.
Consequently, in the case where the elastic sheet members are used instead of the switching plate described in JP-2005-148365-A, with the front ends of the elastic sheet members contacting the guide portion, the second faces of the elastic sheet member cannot be used to direct the sheet toward the attachment belt again. Therefore, when the elastic sheet members of JP-2002-316758-A are applied to the configuration described in JP-2005-148356-A, it is necessary to provide a configuration to forcefully direct the sheet toward the attachment belt so that the sheet can be attached on the belt. As described above, using the elastic sheet members instead of the switching plate may cause a need for an additional configuration for sheet conveyance, thus resulting in cost increase due to increases in structural complexity and number of components.